For example, in a manufacturing process of a semiconductor device or the like, various processes such as an etching process, a film forming process and the like are performed on a semiconductor wafer as a substrate (hereinafter, simply referred to as a “wafer”). As a technique for forming a film on a wafer, a so-called ALD (Atomic Layer Deposition) process may be used. In the ALD process, a film is formed by alternately supplying a source gas to be adsorbed on the surface of the wafer and a reactant gas (also referred to as a reducing gas) reacting with the source gas into a processing chamber in a vacuum atmosphere multiple times and depositing an atomic layer of a reaction product on the surface of the wafer. The source gas and the reactant gas are supplied at different timings to prevent particles from being generated due to gas phase reaction between the source gas and the reactant gas in a region other than the surface of the wafer in the processing chamber. An inert gas is supplied between the supply of the source gas and the supply of the reactant gas. Accordingly, the inside of the processing chamber is purged and an atmosphere in the processing chamber is replaced with an inert gas atmosphere.
Since it is necessary to perform a purge process as described above, the inert gas may be continuously supplied at a predetermined flow rate into the processing chamber during ALD, for example. The inert gas functions as a carrier gas of the source gas or the reactant gas while these gases are supplied and also functions as a purge gas while these gas are not supplied.
Japanese Patent Application Publication No. 2016-023324 discloses a film forming apparatus for performing ALD, in which a bypass channel is provided in a gas channel connecting a processing chamber and a supply source of N2 gas serving as a carrier gas for a processing gas (raw material gas and reactant gas) and a purge gas. In this film forming apparatus, when the processing gas is supplied into the processing chamber, a valve in the bypass channel is closed. When the purge process is performed, the valve is opened to relatively increase the flow rate of N2 gas supplied into the processing chamber.
Japanese Patent Application Publication. No. 2014-198872 discloses a film forming apparatus for performing ALD while increasing the amount of N2 gas supplied into the processing chamber during the purge process, the film forming apparatus including a source gas channel connecting a source gas supply source and the processing chamber, a first N2 gas channel branched from the source gas channel, a second N2 gas channel for supplying N2 gas as a purge gas into the processing chamber, the second N2 gas channel being separately provided from the source gas channel and the first N2 gas channel.
In order to miniaturize a wiring, it is required to perform ALD for ensuring formation of an extremely thin film with good continuity. As a method for ensuring the formation of an extremely thin film with good continuity, there may be considered a method of increasing time per a reactant gas supply step and a method of increasing a partial pressure in the processing chamber by increasing a flow rate of the reactant gas.
However, in the former method, the productivity decreases. If the partial pressure is increased by increasing the flow rate of the reactant gas as in the latter method, it is required to increase the purge time to prevent the generation of particles and, thus, the time required for the film forming process is increased. As in Japanese Patent Application Publication No. 2016-023324 and Japanese Patent Application Publication No. 2014-198872, it is possible to prevent the increase in the purge time by increasing the amount of N2 gas supplied into the processing chamber during the purge process. However, if the flow rate of the reactant gas is high, the amount of reaction by-products deposited on an inner surface of the processing chamber and a gas exhaust system increases and a gas cost increases. Under these circumstances, there is a demand for a technique capable of forming an extremely thin film with good continuity even if the flow rate of the reactant gas is relatively small and also capable of quickly purging the inside of the processing chamber.